The use of thin films in anti-reflection coatings and interference filters is well known. With regard to the former, the thickness and refractive index of the one or more thin film layers is selected to decrease light reflected from a substrate. With regard to the latter, the thickness, refractive index, and/or number of layers is selected to provide the desired filter properties.
More recently, there has been increasing interest in using thin films to provide birefringence in optical components. Birefringence, which is characterized by multiple different indices of refraction, causes light having orthogonal linear polarizations (e.g., s and p polarized light) to propagate with different velocities through a medium. The varying velocity results in a phase difference between the two orthogonal polarizations. This phase difference, which is generally termed retardance, can be expressed as a fraction of a wave, either in degrees or nanometers. In general, the magnitude of the retardance will determine the application of the birefringent element. For example, optical components providing a retardance of about a ½ wave (i.e., a half waveplate) are typically used to change the polarization state of linear polarized light. Optical components providing a retardance of about a ¼ wave (e.g., a quarter waveplate) are commonly used for converting linearly polarized light to circularly polarized light, or in reflection for changing the polarization state of linearly polarized light, and/or in various other applications, such as optical disc pickups. Optical components providing a lower magnitude of retardance (e.g., under 1/10 wave) are typically used to compensate for imperfections in polarization sensitive optical systems (e.g., in LCD projection systems).
In general, birefringent optical components may be characterized as having A-plate, C-plate, or O-plate symmetry. An A-plate is a uniaxial birefringent optical element having its extraordinary axis oriented parallel to the plane of the plate. A C-plate is uniaxial birefringent optical element having its extraordinary axis oriented perpendicular to the plane of the plate (i.e. parallel to the direction of normally incident light). An O-plate is a uniaxial birefringent optical element having its extraordinary axis (i.e., its optic axis or c-axis) oriented at an oblique angle with respect to the plane of the plate. Notably, a C-plate does not provide any net retardation for normal-incident rays (i.e., normal incident light is unaffected by the birefringence). In contrast, rays incident off-axis (i.e., at an angle to the extraordinary axis) experience a net retardation that is proportional to the incident angle. A C-plate is considered to be positive if the retardance increases with angle of incidence and negative if the retardance decreases with angle of incidence. As is referred to in this context, retardance is used interchangeably with retardation, which includes signed phase difference between two orthogonal linear polarization components. A positive C-plate requires positive birefringence and similarly, a negative C-plate requires a negative birefringence.
Birefringent optical components that function as an A-plate are often used to provide/compensate for in-plane retardance, whereas components that function as a C-plate are often used to provide/compensate for out-of-plane retardance. In-plane retardance, expressed as optical path length difference rather than phase difference, refers to the difference between two orthogonal in-plane indices of refraction times the physical thickness of the optical element. Out-of-plane retardance, also expressed as optical path length difference, refers to the difference of the index of refraction along the thickness direction (z direction) of the optical element and one in-plane index of refraction (or an average of in-plane indices of refraction), times the physical thickness of the optical element.
Birefringence in optical components has been traditionally provided by molecularly birefringent crystals, by stretching or bending isotropic materials until isotropy is lost, and/or by applying an electric field to induce anisotropy. As discussed above, there has also been increased interest in using thin-films to provide birefringence in optical components. The use of thin-films is advantageous because it provides an economic alternative, it circumvents reliability problems associated with organic and/or polymeric materials, and it provides additional design flexibility for tailoring for specific applications. For example, while birefringent crystals are highly durable and/or stable compared to their organic and/or polymeric counterparts, the cost of growing and polishing large crystal plates can be significant. In addition, since it is difficult to fabricate a birefringent crystal with a physical thickness less than about 100 microns, these naturally birefringent elements typically are not generally suitable for compensation applications. For example, the C-plate retardance value of a single quartz plate having a thickness of at least 100 microns and a birefringence of 0.009 would be limited to a minimum of 900 nm in the visible region.
Successful attempts at using thin films to provide birefringence have been reported in U.S. Pat. No. 7,170,574 and US. Pat. Appl. No. 20070070276, both of which are hereby incorporated by reference. In these references, thin film coatings are used to fabricate trim retarders for compensating for the inherent birefringence of LCD panels. More specifically, the thin film coatings are used to create a form-birefringent thin-film stack, wherein the optical thickness of the individual layers is much less than the wavelength of the incident light. Conveniently, the form-birefringent (FB) stack, which is highly durable and acts as a −C-plate, can be integrated into an AR coating design (FBAR) to provide a full-function A/−C-plate trim retarder. Unfortunately, due to the layer thickness restrictions, the FBAR can only act as a negative C-plate.